The use of optical devices for the protection against counterfeiting, illegal tampering and product protection in general is now a well established art.
Due to increased fraud and counterfeit, novel anti-counterfeit measures are constantly required. For many years holograms have been the preferred security technology. Meanwhile, this technology is more than 30 years old and therefore well known and widespread. Holographic foils may even be found in every gift shop today. This situation represents a security risk since many people have access to the hologram technology. With the availability of digital hologram printers, the path to easy to use holographic mastering systems has further opened. These printers allow the production of many different types of holograms and a minimal knowledge of holographic set-ups or laser writers is required. Such equipment allows the preparation of masters for the subsequent metal master fabrication and the replication into thin-films in large volumes.
It is thus most desirable to extend the palette of security devices by novel security features, which are clearly distinguishable from holographic devices. Examples of such new devices are alternative optically variable devices (OVD). OVDs are devices that change their appearance as the viewing angle or illumination angle is changed. A subgroup of OVDs are colorshift devices. Colorshift OVDs change their color as the viewing or illumination angle is changed. Prominent representative colorshift OVDs are cholesteric or interference films, including optical devices based on flakes of such films. Both exhibit a pronounced colorshift as the device is tilted away from a perpendicular angle of view. No rainbow colors, a characteristic feature of standard mass-produced holographic devices, are observed in these types of colorshift devices.
Colorshift effects due to the interference of light at thin optical films have a long tradition in the history of modern thin film components (e.g. J. A. Dobrowolski, “Optical thin-film security devices”, in “Optical Document Security” ed. by R. L. van Renesse, Artechouse Boston 1998). Many different compositions of layered thin-film systems are possible. A characteristic reflection spectrum is obtained for instance at normal light incidence. The reflection or transmission spectra are shifted toward the short wave-length side as the incidence angle increases. Multi-layer thin-film systems, often combinations of dielectric and metallic layers, are also possible with dielectric materials only. In this case, thin-films of different index of refraction are required.
Security devices based on either thin interference films or on flakes of such films are commercially available today. Examples can for instance be found in U.S. Pat. No. 5,084,351 and U.S. Pat. No. 6,686,042 of Flex Products, Inc.
Other approaches are scattering devices. The use of isotropic and even more anisotropic scattering effects in OVDs can enhance the optical attractiveness significantly. Especially anisotropic light scattering is a helpful means to generate viewing angle sensitive devices. FIGS. 1.1 and 1.2 illustrate isotropic and anisotropic light scattering respectively.
The reflection at an isotropically structured surface, such as a newsprint or most surfaces encountered in household articles, is such that no azimuthal direction is preferred. As indicated in FIG. 1.1, collimated incoming light 1 is redirected at the scattering surface 2 into new outgoing directions 3 with a characteristic axial-symmetric output light distribution and a characteristic divergence angle 4.
An anisotropically structured surface however reflects light in a pronounced way into certain directions and suppresses light in other directions. In FIG. 1.2, collimated incoming light 1 impinges on an anisotropically scattering surface 5 and is redirected into new outgoing directions 6 with a characteristic output light distribution 7, which depends on the corresponding azimuthal angle 8, 8′.
For the representation of information, a pattern of individual pixels with anisotropic scattering behavior and differing anisotropy direction orientation can be made. In this way, corresponding devices may comprise a patterned anisotropically scattering surface, which represents an image such as a text or a picture or the like. Since the light in a given direction is reflected or suppressed depending on the specific pixel orientation, an image of bright and dark pixels is seen. In addition, these devices exhibit a pronounced change from positive to negative view when they are tilted or rotated. Such patterned surface devices can for example be generated as follows. First, the gray scale image of interest is rastered, that means, the image is split into dark and bright zones with a certain pixel resolution. Then, the dark zones (pixels) are attributed to anisotropically scattering zones of a first orientation direction and the bright zones are attributed to anisotropically scattering zones with a different orientation direction, e.g. perpendicular to the first orientation direction. An illustration of a square of 2×2 pixels with such an orientation arrangement is given in FIG. 2. Pixels 10 and 10′ are oriented in one direction and pixels 11 and 11′ are oriented crosswise. A device with a pattern of pixels arranged like this will appear as positive under a first viewing angle and will flip to the negative as the device is e.g. rotated by 90°.
A known method of manufacturing anisotropic scattering films with patterned anisotropy is described in the international patent application WO-01/29148 of Rolic AG and also, for example, in Ibn-Elhaj et al., “Optical polymer thin films with isotropic and anisotropic nano-corrugated surface topologies”, Nature, 2001, vol. 410, p. 796-799. For the making of the surface structures, use is made of a so called monomer corrugation (MC) technology. It relies on the fact that phase separation of special mixtures or blends applied to a substrate is induced by crosslinking, for instance with exposure to ultraviolet radiation. The subsequent removal of non-crosslinked components leaves a structure with a specific surface topology. The topology may be made anisotropic by the alignment of an underlying alignment layer, and by using a patterned alignment layer, it is possible to create a patterned anisotropically scattering surface topology.
As mentioned above, an interesting and desired feature for many purposes, in particular for the application as security device, are special colors and colorshift effects. In the international patent application WO-2006/007742, it is shown with a single example (Example 5) that based on MC technology it is in principle possible to reach modulation depths, which are deep enough to generate a pastel-colored appearance. However, although the average modulation depth and the average periodicity of MC surface topologies can be tuned by several means, the two parameters can not be made independent from each other. Therefore, and due to the characteristic surface modulation shape induced by the MC technology, the color saturation of MC generated scattering surfaces is limited. More saturated colors, which are essential for many applications, are not possible with corresponding devices.
In the context of optical devices reference is made to US 2003/0072412 A1. In this document an optically active surface structure is disclosed comprising a substrate with a multilayer structure comprising grooves which are positioned between lands. It is noted that the structure disclosed in US 2003/0072412 A1 in principle is a periodic structure, because it is specifically noted that inside each effective period, designated d, the lands are distributed randomly, however within each of these periods the same random pattern is used. So there is a random distribution within one period which is however identically repeated in each period. The structure is thus periodical. Analogous structures are disclosed in DE 10 2004 003 984 A1 as well as in US 2005/0094277 A1.
Equally periodic surface structures are disclosed in US 2005/0219353 A1 in the context of anti-reflective coatings. In spite of the fact that in the text random placement of protruding optical pieces is mentioned, there is no display of a non-periodic arrangement of these optical pieces. On the other hand the actual methods disclosed for making anti reflective coating structures will clearly not lead to structures with protruding optical pieces as given in the displays, i.e. with a constant modulation depth.
US 2003/0011870 A1 discloses a substrate with a light reflection film in which the heights of a plurality of convex portions or the depths of concave portions formed on a base material are specified to be substantially the same. The two-dimensional shapes of the plurality of convex portions or concave portions are specified to be the two-dimensional shapes of independent circles and polygons, or of either of them. In addition, the plurality of convex portions or concave portions are arranged in the direction of the plane on a random basis. The substrate is formed using a mask in which light transmission portions or light non-transmission portions are formed in units of dots, the number thereof being smaller than the number of dot regions. The dots are arranged irregularly in each of the units, and a plurality of units are included.
JP2005-215642 provides a photomask for manufacturing a diffusion reflector having high scattering luminance and a method for manufacturing the diffusion reflector by using the photomask. The photomask has a pattern region where a unit pattern region is laid in a matrix. The pattern region has a rectangular light transmitting part in a plurality of numbers laid in a matrix, a circular minute light transmitting part in a large number regularly or randomly laid to surround each light transmitting part, and a light shielding part surrounding the. Further, a strip portion surrounding the light transmitting part has no minute light transmitting part, and the width of the strip portion is 1 μm to 5 μm.